Then users could click a button to hide australiaJoke2763 once they see it the first time and then they'd never have to see posts with it again.

This would be ideal. No one would care about people cluttering forums with jokes as long as they marked them up properly. Any un-marked or mis-marked joke would get downvoted to hell by all people who had heard it before.

Forums could advertise "JokeDB compliance" as a selling point.

To get things started the only thing you'd need is a really well organized and easy to navigate (easy to find relevant jokes quickly) joke site and buy-in from reddit or any other popular discussion site<joke key="redditPopularJoke1663"> (I think there are other places on the net that people chat, but I'm not sure)</joke>.

Need one of these for memes too. Should be crowdsourced as well as self-tagged; if enough people tag something as a meme (by selecting the tag and clicking a particular button for example) then it would be hidden unless clicked upon.